The integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs, where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs.
Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. For example, gate replacement processes, which typically involve replacing polysilicon gate electrodes with metal gate electrodes, have been implemented to improve device performance, where work function values of the metal gate electrodes are designed to provide different fin-type field effect transistors (FinFETs) with different threshold voltages. Although existing FinFETs providing multiple threshold voltages have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects as IC technology nodes shrink.